Reversible emulsions stabilized by amphiphilic polymers

ABSTRACT

The invention concerns modified hydrophobic polyelectrolytes by amide formation of a hydrophilic skeleton by n-alkylamines whereof the alkyl chain comprises 6 to 22 carbon atoms. Preferably, the amide formation is obtained by di-n-dodecylamine. The hydrophilic skeleton is preferably a sodium polyacrylate or polyacrylic acid corresponding to a statistical acrylate-AMPS copolymer. Said polymers can be used for stabilizing direct or invert emulsions likely to be destabilized or inverted by a modification in the degree of salinity of the aqueous phase or a pH modification. The invention is particularly useful for stabilizing oil drilling fluids or the like in particular drilling, fracturation, acidification or completion fluids.

This application is a Continuation Application of U.S. application Ser.No. 10/892,575, filed on Jul. 16, 2004, now U.S. Pat. No. 7,262,152,which is a Divisional Application of U.S. application Ser. No.09/856,740, filed on Jan. 9, 2002, now U.S. Pat. No. 6,822,039, which isthe U.S. national phase application of International Application NumberPCT/FR98/08497, filed Nov. 23, 1998.

The present invention relates to stabilizing emulsions using amphiphilicpolymers. It is of particular application to the preparation of stableemulsions which can be reversed on demand to enable non-miscible liquidsto be separated and recovered. The invention also relates to fluids usedin drilling, completion or stimulation of hydrocarbon, geothermal oranalogous wells.

An emulsion is an example of a colloidal system formed from twonon-miscible liquids, one being finely dispersed in the other in theform of droplets. Generally, an emulsion prepared merely by stirringtogether the two liquids is not stable, and an emulsifying agent has tobe added to facilitate emulsion formation and to stabilize it.

Certain applications require emulsions which are stable over a longperiod but which can easily be destabilized. This is the case withdrilling mud used during the construction of hydrocarbon or analogouswells. Drilling mud fulfills a plurality of fundamental functions duringconstruction of a well, among them lubricating and cooling the drillingtool, controlling the hydrostatic pressure in the well to counterbalancethe pressure in the traversed formations, and evacuating drill cuttingsto the surface.

Drilling mud is classified into three major categories depending on thenature of its continuous phase:

-   -   Water-base mud, with a continuous phase essentially formed from        water but which may optionally contain additives such as        emulsified oil, salts and water-soluble polymers;    -   oil-base mud, with a continuous phase essentially constituted by        oil, with at most 1% to 15% of dispersed water; and    -   water-in-oil base mud, reverse emulsions which can contain up to        60% water.

Drilling mud also comprises solids such as clays containing additives tocontrol the density of the mud and its suspending power, or solidsoriginating from the drilled formation.

Reverse emulsion types of mud have a multitude of advantages but moreand more often these have to be weighed against environmental problems,in particular for offshore drilling. The mud itself is always recycledbut the cuttings have to be removed after separating them on the surfaceusing mechanical separator means for separating out solids. Under thestrictest regulations, it is permitted to discharge cuttings into thesea only when the cuttings contain less than 1% of organic substances,which amount is greatly exceeded with reverse emulsion type mud becauseof the film of mud which contaminates the cuttings and which cannot beeliminated using the mechanical means employed.

Proposals have therefore been made to “wash” the cuttings beforedischarging them to the sea. However, the surfactants added to stabilizethe reverse emulsion are so effective that the washing water itself isemulsified in the mud, such that the oil is dispersed very little in thewashing water while both the volume and the viscosity of the mudincrease. Adding detergents to destabilize such emulsions has alsoproved to be largely ineffective. Further, such detergents themselvescause environmental problems.

United Kingdom patent GB-A-2 309 240 describes water-in-oil emulsionswhich are reversed when the salinity of the aqueous phase is reducedsimply by adding fresh water or even seawater. This remarkable propertyis achieved by using combinations of ethoxylate type non-ionicsurfactants and sulfonate anionic surfactants as the emulsifying agent.However, such combinations of surfactants cannot produce all of theproperties simultaneously, namely endowing the emulsion with highstability, even at high temperatures, while using additives that arebiodegradable and of low toxicity.

Recently, a number of authors have proposed using amphiphilic polymersas the emulsifying agent. Most of the work has been directed towardscopolymers with polyoxyethylene grafts and has shown that the stabilityof a direct (oil-in-water) emulsion increases with the proportion ofgrafts and with their length. Further, R. Y. Lochhead, in particular inACS Symp. Seris. 462, 101,- 19-91, and in other articles published withhis co-workers, has described hydrophobic modified polyacrylates, with ahydrophilic backbone formed from a cross-linked high molecular weightpolyacrylic acid modified to less than 1 mole % with long chainalkylacrylates or alkylmethacylates or with an undefined proportion ofCarbopol hydrophobic residues. The emulsions obtained with highconcentrations of such hydrophobic modified polyacrylates aredestabilized by adding an electrolyte.

There is a need for particular polymers which can stabilize emulsions.

The present invention provides polyelectrolytes which have been modifiedto render them hydrophobic by amidification of a hydrophilic backbone byn-alkylamines, preferably di-n-alkylamines, the alkyl chains of whichcontain 6 to 22 carbon atoms. Amidification is preferably carried outusing di-n-dodecylamine HN—(C₁₂H₂₅)₂.

The proportion of alkylamines introduced into the hydrophilic backbonemust be such that the modified polymer is substantially insoluble inpure water. Preferably, it is in the range 0.10 to 0.50 moles ofn-alkylamine per mole of hydrophilic polymer.

The hydrophilic backbone is preferably:—a sodium polyacrylate with amolar mass which falls within a wide range; preferably, the mass averagemolecular mass is in the range 50,000 to 2,000,000, more preferably inthe range 100,000 to 1,500,000—or the corresponding polyacrylic acid—ora statistical copolymer of an acrylate and2-acrylamido-2-methylpropanesulfonic acid (AMPS) with a compositionwhich falls within a wide range. Preferably, the statistical copolymercomprises 0.3 to 0.7 moles of AMPS per mole of acrylate.

More generally, the hydrophilic polymer is a homopolymer or copolymerbased on monomers selected from polymers comprising one or moreco-monomers selected from acrylic acid, methacrylic acid or any otheralkyl derivative substituted in the β position of the acrylic acid, oresters of these acids obtained with mono- or polyalkyleneglycols,acrylamide, methacrylamide, vinylpyrrolidone, itaconic acid, maleicacid, 2-acrylamido-2-methylpropanesulfonate (AMPS), styrene-4-sulfonicacid or vinylsulfonic acid.

The polymers containing the carboxylate or sulfonate acid groups can becompletely or partially neutralized by organic bases or metal hydroxidesand are then used in the form of salts of an alkali or alkaline-earthmetal.

The invention also relates to emulsions stabilized by the modifiedpolymers of the invention, for example paints. Depending on the degreeof modification of the starting monomers, the polymers of the inventionare effective as stabilizers for direct or reverse emulsions, theemulsion being able to be destabilized or reversed by reducing thesalinity of the aqueous phase or neutralizing the acid. This phenomenonis used to advantage in fluids employed for petroleum or analogouswells, in particular drilling, fracturing, acidizing, or completionfluids. A reverse emulsion is, for example, destabilized (or reversed)by adding fresh water or at least water which is less saline (seawaterbeing the limiting case), sodium hydroxide, potassium hydroxide, sodiumor potassium carbonate, or sodium or potassium salts, complexing agentssuch as polyphosphates, citrates, ethylene diamine tetraacetic acid(EDTA) or sodium nitrilotriacetate (NTA). Destabilizing the emulsionenables the organic phase (oil) to be recovered for recycling, andenables the mineral waste, such as drilling debris, to be eliminatedsince it is no longer wetted by the oil.

The invention is now described in more detail using the followingexamples which illustrate methods for synthesizing the polymers of theinvention and their emulsion stabilizing properties.

I—Synthesis of Hydrophobic Modified Polyacrylates

a) Polyacrylic Acid Precursors

Two commercially available polymers were used, provided by Polysciencesand Scientific Polymer Products Inc., designated P and PP for thederivative with the highest molecular weight. PP was provided in solidform. P, which is sold in aqueous solution, was diluted to 10% and thenfreeze-dried. Both compounds were then used in solid form.

The following table shows their analyses in their basic form by sizeexclusion chromatography in aqueous solution:

Polymer type: P PP Peak molar mass (g/mol) 46000 700000 Number averagemolar mass 42000 74000 (g/mol) Mass average molar mass 125000 1260000(g/mol) Polydispersity index 3 17

The distribution of the two polymers was very wide, but nevertheless itwas possible to estimate that the degree of polymerization by weight ofpolymer PP was 10 times that of P.

PX is the term used below to designate the derivative obtained when Xmole % of didodecylamine is introducedto graft polyacrylate P, and PPXis the term used to designate the derivative of polyacrylate PP. Thederivatives are said to be moderately grafted if X is over 5 and under40, and highly grafted if X is 40 or more.

b) Synthesis of moderately grafted derivatives

The reaction of amines with carboxylic acids in an aprotic solvent,N-methyl-2-pyrrolidone, NMP, in the presence of dicyclohexylcarbodiimide(DCC) as a coupling agent was used to modify the polyacrylic acid.Consumption of DCC led to the formation of dicylohexyl-urea -DCU.

EXAMPLE Synthesis of Polymer P30

2.27 g of polyacrylic acid (0.03 moles, because the water content was 5%by weight) was dissolved in 60 ml of NMP in a thermostatted bath at 60°C. A first half of the reactants was added: 1.59 g (9×10⁻³ mol) of aminewhich had been dissolved in 13 ml of hot NMP, then 1.39 g (1.35×10⁻²mol) of DCC dissolved in 7 ml of NMP was introduced dropwise into theflask. The reaction medium was stirred vigorously for 4 hours beforeintroducing the second half of the reactants—amine and DCC—using thesame procedure. About 24 hours after the start of the reaction, theflask was cooled in an ice bath. The DCU crystals formed were filteredthrough no 4 fritted glass. The filtrate was then neutralized by adding6 equivalents of 10 M sodium hydroxide with vigorous stirring. Thefiltrate was stirred for 4 hours then filtered through no 4 frittedglass. The precipitate was washed with 20 ml of hot NMP and then withtwice 50 ml of methanol. The polymer was purified using a Soxhletextractor provided with a cellulose cartridge, extracting with hotmethanol.

c) Synthesis of Highly Grafted Derivatives

A method similar to that used for the moderately grafted derivatives wasused, this time adding one equivalent of dicyclohexylcarbodiimide (DCC)and one equivalent of 1-hydrobenzotriazole (HOBT)—with respect to theamine—to increase the yield of the amidification reaction.

Example Synthesis of Polymers P40 and P′40

5.25 g (0.07 moles, because the water content in the polymer was 5%) ofpolyacrylic acid was dissolved in 150 ml of NMP, and stirred for 12hours at 60° C. 4.96 g (0.028 moles) of di-n-dodecylamine(Didodecylamine), 1.89 g (0.028 moles) of HOBT then 2.88 g (0.028 moles)of DCC were successively introduced after prior dissolution in hot NMP.The second portion of the reactants was added in the same manner fourhours later: 4.96 g (0.028 moles) of didodecylamine, 1.89 g (0.028moles) of HOBT then 2.88 g (0.028 moles) of DCC were successivelyintroduced after prior dissolution in hot NMP. The temperature was keptat 60° C. for 24 hours after initial. introduction of the reactants. Thereaction medium was then cooled to 0° C., and the dicyclohexylureacrystals formed were filtered through no 4 fritted glass. The modifiedpolymer was then precipitated by neutralization: 6 equivalents of 10 Msodium hydroxide were added to the filtrate dropwise. After about 12hours of stirring, the suspension obtained was filtered through no4fritted glass, and the polymer was washed with methanol then dried undervacuum at room temperature using a vane pump. The aqueous 10% polymersuspension was dialyzed using a membrane with a cut-off threshold of12,000 g/mol in an aqueous sodium hydroxide solution stabilized at a pHof 9. After several days of dialysis, when the pH of the medium wasstable, the suspension was concentrated and freeze-dried.

For the most hydrophobic derivatives (X>40), a first filtrate in NMP wasrecovered and treated conventionally. This fraction corresponded topolymer PX. A second fraction was recovered by partial precipitationwith dicyclohexylurea. The two compounds were then separated bysuccessive washes with ethyl ether. The solution of the polymer in etherwas concentrated and taken up in NMP. This second fraction, P′X, wasthen treated as for the first fraction.

d) Acidification of Grafted Derivatives

The derivatives obtained in the basic form were changed into their acidform. The polymer, reduced to a powder, was poured into a 0.1 Mhydrochloric acid solution. After 12 hours of vigorous stirring, thesolution was filtered. The precipitate was washed with pure water thendried under vacuum at room temperature.

We shall now describe grafted polyacrylates and grafted polyacrylicacids, it being understood that the polyacrylates tested were sodiumsalts.

e) Analysis of Grafted Polyacrylates

Each compound underwent elemental analysis to determine the respectivepercentages of C, H, N and Na. The ratios

$\frac{\%\mspace{14mu} C}{\%\mspace{14mu}{Na}}\mspace{14mu}{and}\mspace{14mu}\frac{\%\mspace{14mu} N}{\%\mspace{14mu}{Na}}$enabled X to be deduced.

The results are summarized in the following table:

Polymer name P6 P15 P25 P30 P40 P50 P′50 PP50 Degree of 3 12 22 30 35 5148 47 grafting (%) Percentage of 20 15 8 9 9 10 10 10 water, by weightAnalysis of 99 100 99 94 89 63 61 75 modified polyacrylic acidsf) Viscosity in Aqueous Solution

The viscosity in aqueous “solution” of the modified polyacrylates wasstudied for solutions containing 1% of polymer. The least graftedpolymer behaved as an associative hydrosoltible polymer: the alkylgrafts associate together in the hydrophobic zones caused physicalreticulation of the medium and an increase in the overall viscosity withrespect to the precursor polyacrylate, For moderately grafted polymers(more than 10% dialkyl side chains), which were not hydrosoluble, therelative viscosity in water was lower than that of the polyacrylateprecursor. For the most highly grafted polymers, which were stronglyhydrophobic (P50, p′50 and PP50), the relative viscosity in water wasclose to 1.

II—Synthesis of Grafted Aa-Amps Terpolymers

Terpolymers based on AMPS (2-acrylamido-2-methyl-propanesulfonic acid)were prepared in two steps: synthesizing acrylic acid—AMPS copolymers byradical polymerization and hydrophobic modification of these copolymers.In the following examples, acrylic acid/AMPS copolymerizations werecarried out with an ammonium peroxodisulfate (APS) and tetramethylenediamine (TEMEDA) combination as an initiator.

A copolymer obtained with y mole % of AMPS monomer synthesized with nQmoles of initiator was designated PAAMPS-y,nQ where 1Q corresponded to2×10⁻³ moles of APS and 10⁻³ moles of TEMEDA.

Example Synthesis of PA-AMPS-50, 1Q

7.42 g (3.58×10² moles) of AMPS, 2.58 g (3.58×10² moles) of acrylic acidand 0.25 g (2×10² moles) of APS were dissolved in 100 ml of deionizeddistilled water and placed in a flask provided with a magnetic stirrerand in an inert atmosphere, at room temperature. The pH was adjusted to9 by adding sodium hydroxide. After 30 minutes, 0.25 g (10⁻³ moles) ofTEMEDA was introduced. After 4 hours, the polymer was precipitated inacetone and vacuum dried. An aqueous 5% solution was prepared andfiltered over a membrane with a cut-off threshold of 10,000 g/mol. Thedialyzed solution was then concentrated and the aqueous polymer solutionthus obtained was changed into its acid form using an ion exchangeresin. The solution recovered at the column outlet was concentrated andfreeze-dried.

The polymer was grafted using the same procedure as that described forthe hydrophobic modified poly-acrylates. The acid form of the copolymerswas dissolved in NMP then one equivalent—with respect to AMPS—of sodiumhydroxide was added before carrying out the normal grafting procedure.The amidification reaction was carried out in the presence of oneequivalent of DCC and one equivalent of HOBT with respect to the amine.

The grafted AA-AMPS terpolymers were designated as C Z X where Z is thepercentage of AMPS units in moles and X is the rounded mole percentageof didodecylamine, or effective modification.

The effective degree of modification was determined by ¹³C NMRspectroscopy as a function of the number of moles X′ of didodecylamineintroduced for grafting. Because of the imprecise nature of themeasurements, it was decided to use a rounded value for X to designatethe polymer.

Polymer Z X′ ¹³C NMR designation 40 5 4 C-40-5 40 20 10 C-40-10 40 60 40C-40-40 60 20 11 C-60-10 60 40 20 C-60-20 60 60 30 C-60-30III—Emulsions Stabilized by Grafted Polymers in Accordance with theInvention

10 ml volumes of emulsion were prepared by mixing an aqueous phase (purewater or highly saline water with 20% by weight of sodium or calciumchloride), an organic phase constituted by 1,1-hexadecane and 1% graftedpolymer. W4-O6 was the designation given to an emulsion prepared with 4ml of salt water and 6 ml of 1-hexadecane.

The type of emulsion obtained is known to depend on the order of mixingthe components. Thus a precise protocol was used to prepare theemulsions: at room temperature, 100 mg of polymer was stirred for 48hours in the volume of hexadecane. Then the volume of aqueous phase wasadded and the mixture was dispersed by stirring for 3 minutes at 24,000revolutions per minute (rpm)

A—Pure Water—Hexadecane Type Emulsions

Emulsion stabilized by grafted polyacrylates P50 P40 P30 OIL IN P25WATER P15 W109 W208 W307 W406 W505 W604 W703 W802 W901

Emulsion stabilized by grafted polyacrylic acids P50 P40 P30 OIL IN P25WATER P15 W109 W208 W307 W406 W505 W604 W703 W802 W901

Emulsion stabilized by grafted AA-AMPS terpolymers C-40-60 C-60-30 OILIN C-60-20 WATER C-60-10 W109 W208 W307 W406 W505 W604 W703 W802 W901

If the polymer is not too hydrophilic (less than 30% grafting forcharged polymers; acidification of polyacrylates leading toneutralization of charges), it can be seen that for a given stabilizingpolymer, the emulsion could be reversed by changing the volume fraction.

At a given volume fraction, the emulsion could be reversed by increasingthe hydrophobic nature of the stabilizing polymer, i.e., by increasingthe degree of grafting.

B—Saline-Hexadecane Phase Type Emulsions

For these and all of the subsequent tests, a “neutral” composition wasselected with half the volume being saline aqueous solution and half thevolume being oil. After 24 hours, the percentage by volume which hademulsified and the appearance of the emulsion were recorded. Fordroplets of millimeter order, the emulsions were said to bemillimetrically translucent; finer droplets produced cloudy emulsions;finally, droplets of the order of a micrometer resulted in a whiteemulsion.

Emulsion stabilized by grafted polyacrylates Pure water 20% NaCl 20%CaCl₂ P15 80% O/W white 70% O/W translucent P25 70% O/W white P30 70%O/W white P40 80% O/W white P50 P′50 PP50

Emulsion stabilized by grafted polyacrylic acids Pure water 20% NaCl 20%CaCl₂ P15 P25 P30 P40 P50 P′50 PP50

Emulsion stabilized by grafted AA-AMPS terpolymers Pure water NaCl 20%CaCl₂ 20% C-60-10  80% O/W No emulsion white Polymer precipitatedC-60-20  70% O/W at interface C-60-30 100% O/W C40-40  75% O/W whiteIV—Comparative TestsNon Grafted Polymer Precursors

Emulsions were prepared using the same method as above (“neutral”composition, 1% polymer) from a sodium polyacrylate P, not grafted, andAA-AMPS copolymers, with respectively 40 mole % and 60 mole % of AMPS.The appearance and volume were recorded two hours after preparation.

Pure water 20% NaCl 20% CaCl₂ Sodium two distinct two distinctpolyacrylate phases phaeses C-40 two distinct phases C-60 two distinctphases

In all cases, only a direct emulsion could be formed.

Surfactant

Under identical emulsification conditions, emulsions were prepared usingtwo conventional surfactants. Sorbitan monooleate is a non ioniclipophilic surf actant. AOT or sodium bis(2-ethylhexyl)sulfosuccinate isan ionic hydrophilic surfactant.

Pure water 20% NaCl 20% CaCl₂ Sorbitan monooleate AOT 75% O/W two cleartwo clear white phases phasesV—Toxicity of Grafted Polymers of the Invention

The majority of known surfactants used to prepare emulsions are highlytoxic; since the polymers of the invention are only slightly soluble inwater and have high molecular weights, the toxicity can be lower. Thiswas verified by testing the P50 polymer (sodium polyacrylate) using thegrowth inhibition method using single-cell marine algae (AlgaeSkeletonema Costatum).

The concentration which inhibited the growth of 50% of the algaepopulation after 72 hours was more than 10,000 mg/l with polymer P50.With many known surfactants, this same critical concentration is lessthan 10 mg/l, demonstrating the importance of the polymers of theinvention in preparing emulsions in a sensitive medium.

1. A method of formulating a direct emulsion drilling fluid, said methodcomprising: mixing an oleaginous fluid, a non-oleaginous fluid, and apolymeric surfactant, wherein the polymeric surfactant is apolyelectrolyte having a hydrophilic backbone which has been amidifiedby n-alkylamines in which the alkyl chains contain 6 to 22 carbons, andwherein the polymeric surfactant is in amounts sufficient to form adirect emulsion; and wherein the hydrophilic backbone is a sodiumpolyacrylate with a molar mass in the range of 50,000 to 2,000,000. 2.The method of claim 1, wherein the effective degree of modification ofthe polymer is in the range 0.10 to 0.50 moles of n-alkylamine per moleof hydrophilic polymer.
 3. A method of formulating a direct emulsiondrilling fluid, said method comprising: mixing a non-oleaginous fluid,an oleaginous fluid, and a polymeric surfactant, wherein the polymericsurfactant is a polyelectrolyte having a hydrophilic backbone which hasbeen amidified by n-alkylamines in which the alkyl chains contain 6 to22 carbons, and wherein the hydrophilic polymer backbone is ahomopolymer or copolymer based on monomers selected from acrylic acid,methacrylic acid, or any other alkyl derivatives substituted in the βposition of the acrylic acid or esters of these acids obtained withmono- or polyalkyleneglycols, acrylamide, methacrylamide,vinylpyrrolidone, itaconic acid, maleic acid, 2-acrylamido-4-sulfonicacid (AMPS) or vinyl sulfonic acid.
 4. A method of drilling asubterranean well with a drilling fluid, said method comprising: mixingan oleaginous fluid, a non-oleaginous fluid, and a polymeric surfactant,wherein the polymeric surfactant is a polyelectrolyte having ahydrophilic backbone which has been amidified by n-alkylamines in whichthe alkyl chains contain 6 to 22 carbons, and wherein the hydrophilicpolymeric surfactant is in amounts sufficient to form a direct emulsionin which the oleaginous fluid is the continuous phase and thenon-oleaginous fluid is the discontinuous phase, circulating saidemulsion within said subterranean well and drilling said subterraneanwell using said emulsion as the drilling fluid.
 5. The method of claim4, wherein the hydrophilic polymer backbone is a homopolymer orcopolymer based on monomers selected from acrylic acid, methacrylicacid, or any other alkyl derivatives substituted in the β position ofthe acrylic acid or esters of these acids obtained with mono- orpolyalkyleneglycols, acrylamide, methacrylamide, vinylpyrrolidone,itaconic acid, maleic acid, 2-acrylamido-4-sulfonic acid (AMPS) or vinylsulfonic acid.
 6. The method of claim 4, wherein the hydrophilic polymerbackbone is a sodium polyacrylate.
 7. The method of claim 4, wherein thehydrophilic backbone is a statistical copolymer of an acrylate and2-acrylamido-2-methylpropanesulfonic acid.
 8. The method of claim 4,wherein n-alkylamine is a di-n-alkylamine.
 9. The method of claim 4,wherein the n-alkylamine is di-n-dodecylamine.
 10. The method of claim4, wherein the effective degree of modification of the polymer is in therange 0.10 to 0.50 moles of n-alkylamine per mole of hydrophilicpolymer.